Debugging a hung ruby process with GDB

GDB is a great xNix debugger than is hugely useful into tracking down why a ruby process is hung. Below is a very simple how-to.

1. Install GDB

This is different between operating system. On Ubuntu, it is as simple as:

sudo apt-get install gdb

2. Find the hung process id

It will typically be the far left number associated with process you are looking for.

ps aux | grep ruby

3. Point GDB at that process

Once started you will see a lot of text scroll past.

gdb -p <process-id>

4. Print the backtrace

This is the part where you find what you are looking for.

bt

While this may look a little cryptic, it is a C backtrace and will tell you exactly where things are seizing. In this example it was a long running profiling tool that locked things up.

	#0 0x007611e0 in rb_stack_trace (result=0xbfd493ac, max_depth=64) at perftools.c:87
	#1 0x007613af in CpuProfiler::prof_handler (sig=27, signal_ucontext=0xbfd4958c, cpu_profiler=0x76b4e0) at src/profiler.cc:285
	#2 0x007622da in ProfileHandler::SignalHandler (sig=27, sinfo=0xbfd4950c, ucontext=0xbfd4958c) at src/profile-handler.cc:450
	#3 <signal handler called>
	#4 0x08055b78 in catch_timer ()
	#5 <signal handler called>
	#6 0x0018a606 in memcpy () from /lib/tls/i686/nosegneg/libc.so.6
	#7 0x11aebb00 in ?? ()
	#8 0x080573fa in stack_extend ()
	#9 0x0df18c24 in ?? ()

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5. Go forth and fix your code

Now you just need to quit out of GDB; thank the world for its existance and go fix your code.

quit